Field of the Invention
The present invention relates to a thin film transistor having an oxide semiconductor thin film layer as an active layer, and an array substrate.
Description of the Related Art
A thin film transistor (TFT) has been actively developed in which an oxide semiconductor mainly containing zinc oxide (ZnO) is used for a semiconductor layer serving as an active layer. The oxide semiconductor enables a TFT with higher mobility to be formed as compared with an amorphous silicon semiconductor, and thus provides an advantage such that a high-definition panel can be realized and a drive circuit can be formed in a panel.
For example, Japanese Patent Application Laid-Open No. 2007-220817, Japanese Patent Application Laid-Open No. 2012-33836, and International Publication No. 2012/043338 disclose a thin film transistor using an oxide semiconductor layer as an active layer.
When a thin film transistor (hereinafter, occasionally a “TFT”) using an oxide semiconductor layer as an active layer (a channel region, source and drain regions) is used, the thin film transistor has a structure such that source and drain electrodes and the oxide semiconductor layer are directly connected to each other. This is because s difficult to form a region corresponding to a region where impurities are doped into silicon (Si) in order to obtain an ohmic contact layer that is used in a conventional Si-TFT and facilitates contact between source and drain electrodes and Si.
However, a conductive film on the source electrode that partially contacts with the oxide semiconductor layer is oxidized to have high resistance, and thus parasitic resistance is generated. As a result, a desired switching characteristic cannot be obtained in some cases.
In a structure examined as a countermeasure, a top gate type TFT is formed, and resistance of source and drain regions in an oxide semiconductor layer is reduced by hydrogen in an interlayer film and by using a gate electrode and a gate insulating film as masks.
However, in a case of an inverse-stagger structured TFT having a bottom gate that is generally used in an amorphous Si-TFT of an array of a liquid crystal display (hereinafter, occasionally “LCD”), it is difficult to reduce the resistance of the source and drain regions to be formed in the oxide semiconductor layer because the source and drain electrodes are formed between the interlayer film and the oxide semiconductor layer. On the other hand, since a channel region formed between the source and drain electrodes is exposed when the interlayer film is formed, hydrogen is supplied from the interlayer film, the resistance of the channel region is reduced, and thus an off operation is lost.